What this calculator does
Thermal diffusivity describes how fast a temperature change travels through a material, as opposed to how much heat it conducts in total. The two are different: a material can conduct well and still respond slowly if it also stores a great deal of heat.
That distinction explains a familiar experience. Metal feels colder than wood at the same temperature because it draws heat from your skin quickly, and its high diffusivity is a large part of why.
The formula
Divide thermal conductivity by the product of density and specific heat capacity. The denominator is the volumetric heat capacity, which is how much energy a cubic metre must absorb to warm by one degree.
| Term | Meaning |
|---|---|
| Thermal diffusivity (α) | Conductivity divided by volumetric heat capacity, in square metres per second. |
| Volumetric heat capacity | Density times specific heat: the energy needed to raise a cubic metre by one kelvin. |
| Transient conduction | Heat flow while temperatures are still changing, which is the regime diffusivity governs. |
The inputs explained
| Field | What to enter |
|---|---|
| Thermal conductivity (k) (W/m·K) | Thermal conductivity in watts per metre-kelvin. |
| Density (ρ) (kg/m³) | Density in kilograms per cubic metre. |
| Specific heat capacity (Cp) (J/kg·K) | Specific heat capacity in joules per kilogram-kelvin. Water is 4,186. |
When to use it
Predicting how fast something heats through
Diffusivity determines how long a temperature change takes to reach the centre of a body.
Choosing a material for thermal response
High diffusivity equalises temperature quickly; low diffusivity resists and smooths out fluctuations.
Understanding building thermal mass
Materials with low diffusivity buffer daily temperature swings, which is what thermal mass in construction relies on.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does conductivity change the diffusivity?
The same heat capacity with a range of conductivities.
| Thermal conductivity | Thermal diffusivity (mm²/s) | Thermal diffusivity (m²/s) |
|---|---|---|
| 0.6 W/m·K | 0.143335 mm²/s | 1.4333e-7 m²/s |
| 50 W/m·K | 11.9446 mm²/s | 1.1945e-5 m²/s |
| 200 W/m·K | 47.7783 mm²/s | 4.7778e-5 m²/s |
Questions
How is this different from conductivity?
Conductivity describes steady heat flow through a material. Diffusivity describes how quickly a temperature change propagates, which also depends on how much heat the material must absorb along the way.
Why does metal feel colder than wood?
Because it draws heat away from your skin far faster. Both may be at room temperature, but the metal conducts and spreads that heat quickly while the wood barely responds, so your skin cools much more against the metal.
What is thermal mass in a building?
Material with high heat capacity and relatively low diffusivity, which absorbs heat during the day and releases it slowly overnight, smoothing the internal temperature swing.
Why is the unit area per time?
Because it governs how far a thermal front penetrates in a given time, and that distance goes as the square root of diffusivity times time. The units follow from that relationship.
For steady-state resistance to heat flow, see the thermal resistance calculator. For the heat needed to change a temperature, see the specific heat calculator.